Intermodulation, or intermodulation distortion (IMD), is a multi-tone distortion product that results when two or more signals are present at the input of a non-linear device. In such a device, if the applied signals are pure tone frequencies, then intermodulation products will be generated by the system and can be found at the sum and difference frequencies of the originally applied frequencies. More generally intermodulation products will be found at nF0+mF1+pF2+ . . . , where n, m, and p, . . . are integers, and F0, F1, F2, . . . are the applied frequencies.
Intermodulation products can be a significant problem for many electronic systems, particularly communication systems.
FIG. 1 depicts an example for illustrating intermodulation products being generated by a mixing operation of a passive two-port device 100.
In this document, a passive device is understood to refer to a device that exclusively includes passive components such as resistors, capacitors, inductors, signal traces, crystals, acoustic wave elements (including surface acoustic wave (SAW), bulk acoustic wave (BAW), and film bulk acoustic wave (FBAR) devices, etc. A passive device as described herein does not include active or gain components, such as amplifiers, transistors, etc.
As illustrated in FIG. 1, passive device 100 receives an input signal x(t) having a corresponding frequency domain representation X(f) and outputs from an output port 105 an output signal y(t) having a corresponding frequency domain representation Y(f).
In general, X(f) may include substantial components at several different frequencies and/or over a range of frequencies, but for simplification of explanation, it will be assumed for the remainder of this discussion that that X(f) is essentially a single frequency signal at a first frequency F1.
As also illustrated in FIG. 1, an interfering or jamming signal j(t) having a corresponding frequency domain representation J(f) is coupled to output port 105, for example from a device such an antenna that is connected to output port 105. In general, J(f) may include substantial components at several different frequencies and/or over a range of frequencies, but for simplification of explanation, it will be assumed for the remainder of this discussion that that J(f) is essentially a single frequency signal at a second frequency F2.
The present inventors have appreciated that for at least some passive devices, output port 105 may act as a mixing node whereby the first frequency F1 of the input signal is mixed with the second frequency F2 of the interfering or jamming signal. In that case, the signal Y(f) may have frequency components Fi according to equation (1):Fi=±a·F1±b·F2, where a and b are integers, and where aε(0,∞), and bε(0,∞)  (1)
In the cases where a≠0 and b≠0, the resultant frequency component Fi is an intermodulation product, as described above.
In many systems and applications, certain intermodulation products are of greater concern than others. Often, the most important intermodulation products are the 3rd (third) order intermodulation products where a=2 and b=1, and where a=1 and b=2, namely:F3(Lo)=2F1·F2; and  (2)F3(Hi)=2F2·F1,  (3)where here it is assumed that F2>F1.
For example, when F2=1 GHz and F1=900 MHz, then F3(Lo)=800 MHz, and F3(Hi)=1.1 GHz. As can be seen, these third order intermodulation products have frequencies which fall relatively close to the frequency F1 of the original input signal, and therefore may be difficult to separate from the input signal by filtering, and may be received by receivers that are intended to receive the input signal, etc. Other “odd order” intermodulation products such as 5th (fifth) order (e.g., 3F2−2F1 and 3F1−2F2), 7th (seventh) order (e.g., 4F2−3F1 and 4F1−3F2), etc. and higher order intermodulation products may also include frequencies that are relatively close to the original frequency of the input signal, however the magnitude of these products is typically substantially less than the magnitudes of the third order IM products.
FIG. 2 depicts an example for illustrating intermodulation products being generated by a mixing operation of a passive three-port device, namely a duplexer 200.
As illustrated in FIG. 2, duplexer 200 receives at a transmit input port 201 a transmit signal xT(t) having a corresponding frequency domain representation XT(f) which passes through a transmit filter of duplexer 200 and is provided to a common port 205 thereof. In many systems, common port 205 may be connected to an antenna, and so common port 205 is often also referred to as an antenna port. Meanwhile, duplexer 200 receives at common port 205 a signal zR(t) having a corresponding frequency domain representation ZR(f) which passes through a receive filter of duplexer 200 and is provided to a receive port 203 thereof as a receive signal xR(t)/XR(f).
In general, both XT(f) and XR(f) may include substantial components at several different frequencies and/or over a range of frequencies, but for simplification of explanation, it will be assumed for the remainder of this discussion that the portions of XT(f) and XR(f) that are of interest are each essentially single frequency signals at first and second frequencies F1 and F2, respectively.
As also illustrated in FIG. 2, an interfering or jamming signal j(t) having a corresponding frequency domain representation J(f) may also be coupled to common port 205, for example from an antenna that is connected to common port 205. In general, J(f) may include substantial components at several different frequencies and/or over a range of frequencies, but for simplification of explanation, it will be assumed for the remainder of this discussion that that the portions J(f) that is of interest is essentially a single frequency signal at a third frequency F3.
The present inventors have appreciated that for at least some three-port devices, common port 205 may act as a mixing node whereby the first frequency F1 of the transmit signal is mixed with the third frequency F3 of the interfering or jamming signal. There may also be “internal” mixing nodes which are internal to duplexer 200 and whose mixing products could pass through to common port 205. In that case, XR(f)) appearing at receive port 203 may include intermodulation products of F1 and F3, i.e., ±a·F1±b·F3, where a and b are integers, and where a (0, ∞), and b (0, ∞).
Some of the frequency components comprising these intermodulation products may fall within the reception band—a band which is being received by the receiver circuitry that may be connected to receive port 203. In that case, these intermodulation products become interfering signals for reception of the desired signal xR(t)/XR(f)). To address this problem, some existing apparatuses may try to use filtering to eliminate or reduce the magnitude of the jamming signal. For example a notch filter may be placed at the ANT port of such a duplexer to pass the transmit and receive signals while notching out (blocking) the jammer signal. However, when the jamming signal is located close in frequency to the desired signal(s) to be processed, the required notch filter—in order to reject the undesired jammer—will typically have a high insertion loss, which is undesirable, and in some cases will also give rise to its own 3rd order IM generation. Another complicating factor is that the notch filter may need to be tunable to be able to reject the undesired jammer while passing the desired transmit and receive signals. Such a notch filter would need to be simultaneously tunable, linear, and highly frequency selective.
What is needed, therefore, is an arrangement for reducing, removing, or eliminating one or more intermodulation products that may be generated by a mixing operation of a passive device without the use of a high insertion loss filter.